Cytogenetic and interspecific hybridization in Lathyrus(L.)

Abstract

The genus Lathyrus is widespread in the temperate regions of both old and new world and is of considerable economic importance. L. latifolius, a perennial species, has a history of cultivation as an ornamental, however it lacks the variation in flower colour. In L. Iatifolius it is likely that selection and recombination of mutants will produce a wide range of flower types but a more rapid rate of success may be achieved via interspecific hybridization. Interspecific hybridizations in the genus Lathyrus have also been reported to be more successful between species that are taxonomically closely related, and bear similarities in their karyotypes. This thesis reports interspecific hybridization for the improvement of the perennial pea and a series of cytogenetic studies on Lathyrus species. High resolution karyotype analysis carried out on 20 species of Lathyrus belonging to section Lathyrus indicated that the superficial similarities of chromosome number and karyotype mask significant differences in heterochromatin content and distribution, suggesting that chromosome repatterning, sequence amplification and deletion have accompanied speciation in Lathyrus. Genome sizes were measured in 24 species of Lathyrus using flow cytometry in an attempt to correlate this character with the amount of heterochromatin' degree of chromosome pairing in hybrids and general crossability of the species. A greater than two-fold variation was observed. Significant intraspecific variation was observed for the first time in five specie s of Lathyrus. The distribution of DNA amounts within the section Lathyrus appears to be continuous. Interspecific hybridization using conventional techniques, performed between five perennial species (L. latifolius, L. heterophyllus, L. sylvestris, L. cirrhosus and L. mulkak) and one yellow flowered annual species (L. gorgoni) of Lathyrus in an attempt to produce an improved variety of the perennial pea L. latifolius were unsuccessful. Observations of pollen tube growth following cross pollination were indicative of pre-fertilization barriers in operation hence L. gorgoni does not appear to be closely related to any of the perennial species. The reciprocal crosses L. latifolius x L. heterophyllus and L. latifolius x L. sylvestris were successful. Consequently, L. latifolius appears to be closely related to L. heterophyllus and L. sylvestris. None of the hybrids produced had novel flower colours. Meiotic analysis indicated high chiasma frequency accompanied by low pollen fertility in four interspecific hybrids between L. latifolius and two other perennial species (L. heterophyllus and L. sylvestris). Synaptonemal complex spreads of interspecific hybrids of Lathyrus are reported for the first time and complete pairing was observed. Therefore the cause for this reduced fertility in some of the hybrids has been attributed to segregation of genetically unbalanced combinations in the hybrid. Silver staining in Lathyrus is not specific for NOR localization. This is further supported by in situ hybridization using a ribosomal probe. Earlier studies and present studies indicate the presence of secondary constrictions in different positions in L. chloranthus and L. chrysanthus. FISH using ribosomal probe confirmed the presence of eight sites in L. chrysanthus and six in L. chloranthus.